1. Field of the Invention
This invention relates to apparatus and method for separating immiscible fluid phases of different specific gravities and, more particularly, to such apparatus and method which centrifugally separate flowing immiscible fluid phases.
2. Setting of the Invention
In the oil and gas industry, oil/water separation vessels have been designed in numerous geometries in an attempt to improve oil/water separation efficiency. Unfortunately, volumetric utilization efficiencies as low as 19% have been observed with standard tank designs using laboratory tracer studies. Even the best tank designs often achieve utilization efficiencies of less than about 60%.
One tank design that has a relatively high volumetric utilization efficiency is a downflow, center-outlet vortex separator developed by Amoco Production Company. Please see U.S. Pat. No. 4,619,771. This vortex separator achieves relatively high volumetric utilization efficiency provided the flow rate is within a relatively narrow range.
A similar, known cyclonic device that uses centrifugal force to separate oil and gas is described in Surface Production Operations, Volume I, Arnold and Stewart, Gulf Publishing: 1986. The Arnold and Stewart device uses downflow, i.e., the inlet is at the top of the vessel, and includes a cyclonic chimney or a tangential fluid race. Gas exits from the top of the inner cyclonic chimney and the oil exits out the bottom of the outer tank, i.e., the separated oil and gas exit at opposite ends of the chimney and the gas exits at the same end of the chimney as the inlet; therefore the separating gas must flow countercurrent to the bulk stream.
Another known device which uses a cyclonic flow field to separate two liquids is commonly called a hydrocyclone. In the known hydrocyclones, the coalesced oil stream flows in a direction opposite or countercurrent to the direction of the bulk water flow, i.e., the separated overflow oil and underflow water streams exit the conventional hydrocyclone at opposite ends of the hydrocyclone. The oil exits the hydrocyclone at the same end as the inlet. As a result of this countercurrent flow pattern, the inlet feed pressure, the under flow back pressure, and the overall pressure drop between the inlet and outlet streams are all relatively large. Also, the known hydrocyclones operate at an angular velocity which results in large shear forces in the fluid. The large shear forces can prevent coalescence of the oil and can actually break the oil into smaller droplets, which are then more difficult to force to the center of the hydrocyclone and thus be separated.
In view of the previously discussed problems, there is a need for an apparatus and method for separating flowing immiscible fluid phases of different specific gravities, such as oil and water, which will (a) have a high volumetric utilization efficiency; (b) take advantage of the bulk fluid velocity in achieving separation; (c) facilitate coalescence of the dispersed oil phase or similar dispersed fluid phase; (d) be mechanically simple and may be inexpensively fabricated; (e) operate efficiently at high and low flow rates and over a wide range of flow rates; (f) be easily retrofitted to an existing separating system and/or tank; (g) have the coalesced oil stream and the bulk water stream flow in the same or concurrent direction and exit from the same end of the separator so that the inlet feed pressure, outlet back pressure, and overall pressure drops between the inlet and outlet streams are significantly less than in known hydrocyclones; and (h) create smaller shear forces within the separator in order to provide better coalescence and separation.